Aromatic polyesters are known to be heat-resistant high polymers. However, most of the conventional polyesters are difficult to process, and their application is limited.
Liquid crystalline polyesters having excellent processability and melt anisotropy have been and are now widely studied, as described in an extensive literature, e.g., U.S. Pat. Nos. 4,161,470, 4,219,461, 4,256,624, 4,279,803, 4,299,756, 4,318,841, 4,318,842, 4,330,457, and 4,337,190, Japanese Patent Publication No. 40976/83, Japanese Patent Application (OPI) Nos. 136098/78, 43296/79, 87422/82, 62630/83, 91812/83, 91816/83, and 85733/84, etc. (the term "OPI" used herein means "unexamined published application").
Aromatic polyesters, though generally regarded as excellent in flame retardance, have a limiting oxygen index (hereinafter described) of about 40 at the highest, that does not always provide sufficient flame retardant properties. Further, they should be processed at high temperatures under high pressure because of their very high melting points and high melt viscosities. Exposure to high temperatures for long periods of time is not only unfavorable in view of decomposition of the polyesters, but also economically disadvantageous.
On this account, development of the liquid crystalline polyesters having excellent flame retardance and melt processability have generated much interest, and many proposals have hitherto been made.
Nevertheless, conventional liquid crystalline polyesters as described in the above-described literature are still insufficient in flame retardance and also require high temperatures, usually above 330.degree. C., for melt processing. It has thus been considered extremely difficult for polyesters to have both good melt processability and good flame retardance simultaneously.